The US secretary of state, John Kerry, on Thursday expressed cautious optimism that a deal could be reached to guarantee the destruction of Syria's chemical weapons, as he began talks on the issue in Geneva with his Russian counterpart, Sergey Lavrov.

As the talks got under way in Geneva, the United Nations announced that Syrian president Bashar al-Assad had signed a legislative decree making his country party to the chemical weapons convention.

"In their letter, the Syrian authorities have expressed their commitment to observe the obligations entailed by the convention even before its entry into force for Syria," a UN spokesperson said. "Given recent events, he hopes that the current talks in Geneva will lead to speedy agreement on a way forward which will be endorsed and assisted by the international community."

Kerry insisted that the threat of military action against Syria, over a chemical weapons attack in the country on 21 August, remained an option. He also rejected Assad's suggestion that he would have 30 days, under the standard terms of the chemical weapons treaty, to declare his stockpiles.

"We believe there is nothing standard about this process at this moment because of the way the regime has behaved," Kerry said. "And the words of the Syrian regime, in our judgment, are simply not enough."

The joint press conference between Kerry and Lavrov marked the start of an uncomfortable alliance between the US and Russia, after the two countries unexpectedly converged around a proposal to disarm Syria of its chemical stockpiles. Delegations of Russian and American technical experts are in the Swiss capital to discuss the mechanics of the disarmament plan, which would involve Syria identifying the locations of its stockpiles and handing over control to the international community.

Washington is seeking to shift responsibility for the success of any weapons destruction programme to Moscow. US officials argue that the plans for dealing with Syria's chemical weapons were put forward by Russia, and says its "prestige" is now at stake. "This is not a game. It has to be real," Kerry said in Geneva.

Earlier on Thursday, Assad told Russian television Syria would submit documents to the UN for an agreement governing the handover of its chemical arsenal. "Syria is placing its chemical weapons under international control because of Russia. The US threats did not influence the decision," Interfax said, quoting the state-run Rossiya-24 channel's yet-to-be-aired interview.

The White House and State Department both indicated on Thursday that the US was treating Assad's promise with scepticism. In Geneva, Kerry said: "It is too early to tell whether these efforts will succeed, but the technical challenges of trying to do this in the context of the civil war are obviously immense. But despite how difficult this is, with the collaboration of our experts, and only with the compliance of the Assad regime, we do believe there is a way to get this done."

A senior state department official accompanying the Kerry delegation said the US and Russia both had technical expertise in chemical weapons destruction because of the two-decade process – which is still ongoing – to destroy cold war-era stockpiles.

"We also have experience in doing destruction of chemical weapons in the face of a resistant environment," the official said. "That is Iraq in the 1990s. And we have recent and ongoing experience, successful experience, in the case of a cooperative government. That is Libya, which we expect within the next few months will complete the destruction of its stockpiles left over from the previous regime."

The senior official spoke on the condition of anonymity, as occurs routinely in US administration briefings.

Kerry said Syrian weapons destruction would need to be credible, comprehensive, verifiable and implemented quickly, and added that there would need to be "consequences" for Assad if he did not comply with his obligations. He did not specify whether such consequences would involve the use of force against Syria.

However, he also said that the credible threat of military action had forced Syria into conceding that it possessed chemical weapons and agreeing to their destruction, and maintained that the threat of strikes remained. Lavrov said that the securing of Syria's chemical weapons "will make unnecessary any strike against the Syrian Arab Republic".

After Kerry spoke, Lavrorv appeared to admonish him for making political comments. "Diplomacy likes silence," Lavrov said. Kerry did not hear the translation and asked for it to be repeated. Lavrov said in English: "It's okay, John." Kerry laughed and replied: "You want me to take your word for it. It's a little early for that."

Aside from the jokes, Kerry adopted softer language than has recently been used by the Obama administration, saying only that "force might be necessary" to deter Assad from using his weapons if the diplomatic route failed.

The question over whether force could be authorised, in the event that Syria did not comply with chemical weapons actions, is at the centre of negotiations at the UN in New York, where a possible resolution is being drafted.

In the article, Putin welcomed the new diplomatic initiative over Syria but condemned a US tendency toward unilateral "brute force" and sharply criticised America's belief in its own "exceptionalism". The article caused consternation in Washington.

The Republican senator John McCain said the article was "an insult to the intelligence of every American", while the Democratic chairman of the Senate foreign relations committee, Robert Menendez, said of reading the piece: "I almost wanted to vomit."

The US administration chose more diplomatic language. A State Department deputy spokeswoman, Marie Harf, said the time had come for Putin to "put forward actions now, not just words".

The White House spokesman Jay Carney said: "The important thing is that both in his op-ed and in his statements and actions, president Putin has invested his credibility in the transfer of Assad's chemical weapons."

He added: "[Nevertheless] there's a great irony in the placement of an op-ed like this because it reflects the truly exceptional tradition in this country of freedom of expression – something on the decrease in the last dozen or so years in Russia."

Carney said the White House would not tolerate any delay by the Syrian government and would continue to provide military assistance to rebel groups throughout the process.